Lithium batteries have low internal resistance. Unlike Lead Acid batteries, Lithium batteries, if run down, could accept the full output capacity of the alternator. Therefore the battery should be sized based on the alternator size. If two Lithium batteries are installed, one big and one small, the small battery could be charged at too high of a rate. For instance, a 60 amp alternator could pump a full 60 amps into a 3 amp-hour Lithium battery if the battery was completely run down. To prevent that, EarthX recommends putting a diode in series with the charging circuit. The diode should have a voltage drop of 0.5 (Schottky diode). Lower charging voltage results in less current. Read all about it in this document by EarthX:
https://earthxbatteries.com/dual-bus-lithium-battery-design

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I don't understand the physics of this, Joe. If I have a small, drained lithium battery hanging on a 60amp alternator, it could have the full 60 shoved down its throat. If I hang a larger lithium beside it, the two would share the current in some unknown proportion based on relative internal and installation resistance. If I hung 10 lithium batteries on that same alternator, they *might* protect each other to the point of the pilot not ever having anything to worry about.
At least, that is how I understand the physics. I guess what I don't understand is hw does having a larger lithium hanging in parallel to a smaller one result in more risk?

Lithium batteries have low internal resistance. Unlike Lead Acid batteries, Lithium batteries, if run down, could accept the full output capacity of the alternator. Therefore the battery should be sized based on the alternator size. If two Lithium batteries are installed, one big and one small, the small battery could be charged at too high of a rate. For instance, a 60 amp alternator could pump a full 60 amps into a 3 amp-hour Lithium battery if the battery was completely run down. To prevent that, EarthX recommends putting a diode in series with the charging circuit. The diode should have a voltage drop of 0.5 (Schottky diode). Lower charging voltage results in less current. Read all about it in this document by EarthX:

I understood the risk was the to the small one, with the possible misunderstanding by the installer that the presence of the large battery protects the small, which it doesnâ€™t.
On May 8, 2018, at 09:39, Ernest Christley <echristley(at)att.net (echristley(at)att.net)> wrote:
I don't understand the physics of this, Joe. If I have a small, drained lithium battery hanging on a 60amp alternator, it could have the full 60 shoved down its throat. If I hang a larger lithium beside it, the two would share the current in some unknown proportion based on relative internal and installation resistance. If I hung 10 lithium batteries on that same alternator, they *might* protect each other to the point of the pilot not ever having anything to worry about.

At least, that is how I understand the physics. I guess what I don't understand is hw does having a larger lithium hanging in parallel to a smaller one result in more risk?

Lithium batteries have low internal resistance. Unlike Lead Acid batteries, Lithium batteries, if run down, could accept the full output capacity of the alternator. Therefore the battery should be sized based on the alternator size. If two Lithium batteries are installed, one big and one small, the small battery could be charged at too high of a rate. For instance, a 60 amp alternator could pump a full 60 amps into a 3 amp-hour Lithium battery if the battery was completely run down. To prevent that, EarthX recommends putting a diode in series with the charging circuit. The diode should have a voltage drop of 0.5 (Schottky diode). Lower charging voltage results in less current. Read all about it in this document by EarthX:
https://earthxbatteries.com/dual-bus-lithium-battery-design

The size of any installed Lithium battery must be chosen so that it is capable of accepting the full output of the alternator. If it is desired to use a smaller Lithium battery that can not accept the full alternator output, then some means must be used to limit the charging current to that small battery if the battery happens to be almost discharged. You are right that if there are always other aircraft loads using current, then the full alternator output is no longer available to charge a battery. I guess I did not word my previous post very well and it is confusing. Two different sized Lithium batteries in parallel do not result in more risk. The only risk is when a Lithium battery is charged with more current than it is rated for. If the battery happens to be completely run down, then it will accept the full alternator output which might be too great for the size of battery. One way to limit the charging current is to install a series Schottky diode. The EarthX document (that I provided a link to in my original post) explains it a lot more clearly than I can.

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Even if only one Lithium battery is installed in the aircraft, the size of that battery must be such that it can accept the full alternator output. If the installer bases the battery size only on its ability to crank the engine without considering the size of the alternator, then a large alternator might force too much current into the Lithium battery and damage it. (Fire)

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Every battery has a certain "internal resistance -IR" build in due to its chemistry, connectivity inside, age, temperature, etc., etc. This resistance is variable and very low; in milliohms, but, it is there.

Suppose the smaller of the two batteries has a lower IR than the larger one. The alternator is only looking at the resistance of its load; it really doesn't know that it is a battery, only a load. And, since the smaller in this case may have a lower load IR resistance, the majority of the current is headed into the small battery first. At least, until its IR changes and goes higher, then, current is deflected or shared with the other battery in parallel..... In the case of a much discharged small battery, the current going in could exceed its capability to cope. . . .

[quote] ---

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The issue wouldn't be that the smaller of two Li batteries would have lower inherent internal resistance (it shouldn't), but that the smaller might be fully discharged and the larger be fully charged. In that situation, the lion's share of charge current would flow to the small battery.

Having said that, it raises one of my gripes with the EarthX products. They are supposed to have a robust battery management system built in. That battery management system supposedly controls charging, up to and including cell balancing. The BMS sits between the charging source and the cells of the battery. If it's doing its job, *the BMS* should be controlling charge current into each cell; *not* the generating source. Telling us that a too-large alternator can damage their battery is admitting that the BMS... isn't.
Rant mode off....
Charlie
On Tue, May 8, 2018 at 10:34 AM, David Lloyd <skywagon(at)charter.net (skywagon(at)charter.net)> wrote:
[quote] Every battery has a certain "internal resistance -IR" build in due to its chemistry, connectivity inside, age, temperature, etc., etc.Â This resistance is variable and very low; in milliohms, but, it is there.
Â
Suppose the smaller of the two batteries has a lower IR than the larger one.Â The alternator is only looking at the resistance of its load; it really doesn't know that it is a battery, only a load.Â And, since the smaller in this case may have a lower load IR resistance, the majority of the current is headed into the small battery first.Â At least, until its IR changes and goes higher, then, current is deflected or shared with the other battery in parallel..... In the case of a much discharged small battery, the current going in could exceed its capability to cope. . . .
Â

[quote] ---

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If it is desired to use a smaller Lithium battery that can not accept the full alternator output, then some means must be used to limit the charging current to that small battery if it happens to be almost discharged.
Hi all,

Isn't that what the Battery Management System is for ?
I would have thought that every Lithium battery *must* have such a
device to monitor and manage the charging of each and every element in
the battery, and to prevent overcharging.
The alternator doesn't directly "see" the battery internal resistance,
but rather the BMS interface.
Last year when I ordered two EarthX batteries for installation on a new
aircraft, the - knowledgeable - sales person did not express any concern
over the type or rating of the alternator.
Bottom line, is this a real issue ?

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